The wall doesn't make sense from a practical standpoint - there isn't enough illegal immigration over the southern border to justify billions of dollars to wall it off entirely, where existing security measures and changes in immigration policy might have a better effect.
It does, however, make sense as a symbol of right-wing xenophobia and populist defiance against globalism. If Trump's supporters are focused on the wall, maybe they won't notice any of the other promises he's not keeping.